This type of security sensor device is known, in which an infrared beam transmitter and an infrared beam receiver are arranged at respective opposite ends of a linear alert regions and, while an infrared beam travels from the infrared beam transmitter towards the infrared beam receiver, an entry of a human body into the alert region can be detected once the human body intercepts the infrared beam then traveling from the infrared beam transmitter towards the infrared beam receiver. The infrared beam transmitter and the infrared beam receiver in the security sensor device are of the substantially same appearance with each other (see, for example, the Japanese Laid-open Patent Publication No. H10-039043).
The infrared beam transmitter or receiver such a security sensor device is known, in which a hood or a step is provided to prevent the sky light from impinging upon an optical lens of the beam transmitter and receiver. Accordingly, a portion of a light permeable surface of the cover, through which light is allowed to enter the optical lens, is suppressed from undergoing a radiative cooling as it is shielded from the sky, where a temperature is low, and, therefore, during the winter, a frost is prevented from depositing on the light permeable surface of the cover under the influence of radiative cooling to thereby avoid cutting off the infrared beam by the deposited frost.